Combustion times and emission profiles of micron-sized aluminum particles burning in different environments

Optical signatures and combustion times for micron-sized aluminum particles are measured in oxidizing environments including nitrogen mixtures with oxygen, carbon dioxide, and water. Particles in a room temperature oxidizing gas stream are fed into a CO₂ laser beam where they are ignited. Prior to entering the CO₂ laser beam, each particle crosses a second, low energy laser beam and produces a scattered light signal used to determine the particle size in real time. The correlation between the measured particle sizes and their burn times produces an experimental trend that is compared to various correlations reported in the literature. In addition to the burn time measurements, detailed optical signatures are recorded for micron-sized aluminum particles burning in different environments. For aluminum burning in water vapor, the optical signature of the particle is substantially weaker than in other environments, possibly indicating a primarily surface oxidation. It is shown that semi-empirical and widely used τ_b-Dⁿ expressions for the particle burn time, τ_b, as a function of its diameter, D, are inaccurate for the conditions that are different from those used to establish respective trends initially. It is observed that the effect of oxygen concentration on combustion time of micron-sized aluminum particles is weak when oxygen concentrations exceed 21%. Combustion times increase substantially for lower oxygen concentrations. Aluminum particle combustion times are substantially longer than predicted for experiments with water and carbon dioxide oxidizers. For all environments, the observed effect of particle size is relatively weak and the exponents in the descriptive Dⁿ relations appropriate for the current experiments vary approximately from 0.3 to 1. The exponent close to 0.3 adequately describes current results for those oxidizing environments for which heterogeneous reactions appear to dominate.